Since at least the early 1980's there has been a substantial increase in the need for simple and efficient tamper-evident plastic closures for application to the necks of various bottles and containers. This need has grown geometrically with the rash of tampering which has occurred in the market place in which foreign substances have been placed inside containers prior to purchase. There has thus been a very heavy demand for closures which can be used to visually identify those containers which have previously been opened, so that the consumer will be forewarned against purchasing such products.
In response to this extremely high demand an extremely large number of such tamper-evident plastic closures have been developed. One of the most significant developments in the field of tamper-evident plastic closures is exemplified by U.S Pat. Nos. 4,343,408 and 4,461,390, which relate to a highly successful such closure which has been adopted for use in various commercial environments. These patents, which are assigned to the assignee of the present application, include such a closure which has a depending lower skirt portion as well as an intermediate side wall portion thereabove, and which further includes an inwardly projecting bead on the inside surface of the depending lower skirt portion which is adapted to engage the lower end of an annular collar portion of the container to which the closure is to be applied. A frangible portion, such as a plurality of frangible bridge members, is located along the inner surface of the cylindrical side wall, and the side wall portions both above and below these bridges are substantially in alignment with each other, and preferably also have substantially uniform thicknesses. This permits the thin and flexible bridges to flex and bend during application of the closure onto the container without breaking. This, in turn, is facilitated by the wall portions above and below the frangible portions coming together or contacting each other so as to bear the load during application, again to prevent premature fracture of these bridges. Furthermore, upon the subsequent removal of these closures the same bridges are easily broken, and the depending lower skirt portion of the closure remains on the container after the upper portion of the closure has been removed therefrom. Improvements in this device such as that in U.S. Pat. No. 4,461,390, have also included portions of the upper and lower wall above and below the bridges being parallel to each other to further facilitate acceptance of the load thereacross.
Another prior tamper-evident closure is disclosed in U.S. Pat. No. 4,613,052. In this patent the inwardly projecting bead of the devices shown in the patents discussed above are described as an annular stop ring 20, which when the cap is initially formed takes the configuration shown in FIG. 2 of the '052 patent. Furthermore, when this stop ring is inverted, it can extend inwardly and upwardly relative to the indicator band 15 or lower depending skirt portion of the closure, in the manner shown in FIGS. 3 and 5-7 of this patent. As this patentee specifies, the stop ring 20 has a free end 21 which is somewhat thicker than the thickness of the connection between the ring and band 15. As can best be seen in FIG. 6 of the '052 patent, after the closure has been applied to a container, stop ring 20 is positioned with the upper free end 21 of the stop ring 20 in engagement with the tapered surface 29 of the container finish, and also in close proximity to the bridge 16. In this manner, when the closure is removed, stop ring 20 cannot pass over the bead 26, and the bridges thus fracture.
The overall closure shown in the '052 patent, however, does not fully and consistently perform the function for which it was primarily designed. That is, it is most important that these closures can be easily applied to the container without premature fracture. However, in the past the problem has continued to arise that when this is accomplished on a consistent basis, it then becomes more difficult to consistently insure that upon removal of these closures fracture occurs in every case, since if this does not happen then the basic purpose of these closures will not be realized. The search has therefore continued for improved tamper-evident closures of this general nature, which are not only less expensive, but which, most importantly, have greater reliability, so that the closure can be easily applied to the containers while the avowed purpose of tamper-evidence is still realized in essentially every case.